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- Molecular formula: C4H4
- Molar mass: 52.075
- CAS Registry Number: Not available
- Appearance: Not available
- Melting point: Not available
- Boiling point: Not available
- Solubility: Not available
- Safety sheet: Not available
- Spectra: Check on SDBS. Add Spectra (Help).
Cyclobutadiene is the smallest [n]-annulene (-annulene), an extremely unstable hydrocarbon having a lifetime shorter than five seconds in the free state. It has chemical formula 44. It is believed to be in equilibrium between a pair of rectangular/nonplanar ground states and a square, excited triplet state based upon theoretical calculations, The singlet ground state has a rectangular structure. The first excited state is a triplet with a square geometry. The rectangular structure is consistent with the existence of two different 1,2-dideutero-1,3-cyclobutadiene stereoisomers. This indicates that the pi electrons are localized and therefore not considered to be aromatic. Cyclobutadiene is far from stable, it is highly reactive and has a very short lifetime. Cyclobutadiene dimerizes at temperatures above 35 K by a Diels-Alder reaction. The monomeric form has been studied at higher temperatures by trapping with matrix isolation in a noble gas.
1,3-Cyclobutadiene (IUPAC Name); Cyclobuta-1,3-diene
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